and penetrate into Serbian territory but due to the exhaustion of attacking forces, the following attacks met with less success. The Serbian command was concerned with these developments and decided to reinforce its troop with units from Macedonia, which weakened their advance there and allowed the Bulgarian 4th Army to contain it. The Left Group of the 3rd Army however was ordered to retreat back to the border because it was considered needless to expose it to battle with the fresh Serbian reinforcements. The advance of the Right Group began on 24 June; after a short struggle the Serbian vanguards were pushed back towards Pirot and the Bulgarians took up favorable positions on the high ground overlooking the field around the fortress from which their artillery could support any further attacks with great efficiency. On 30 July, the Group was ordered to dig in and repel the Serbian attacks, that had mainly reconnaissance purpose. On that day, however, the threat to the 1st Army's rear posed by the Romanian invasion of Northern Bulgaria led the Bulgarian high command to order the retreat of that army from Knjaževac, which it had recently occupied. This development made the planned capture of Pirot impossible and left the 3rd Army defending the positions it had so far won until the end of the war. As stipulated by the Treaty of Bucharest (1913), the Bulgarian 3rd Army was demobilized on 29 July. Commanders Lieutenant General Radko Dimitriev (17 September 1912 – 20 June 1913) Major General Racho Petrov (20 June 1913 – 10 August 1913) First World War Two years after the end of the Second Balkan War Bulgaria entered World War I on the side of the Central Powers. General mobilization was declared on 9 September (23 September) with a special royal decree. In accordance the Third Army was activated on 12 September and was placed under the command of Lieutenant General Stefan Toshev and his chief of staff, Colonel Stefan Popov. On 15 September the general issued an order determining the order of battle as follows: On 21 September Toshev received instructions from the chief of staff of the Bulgarian Army General Konstantin Zhostov to deploy his army for the defense of the Romanian border and the Black Sea coast in accordance with the high command's plan for war against Serbia and Greece. This task remained unchanged even after the declaration of war